Synchronization is fundamental in most telecommunication systems, e.g. telecommunication systems based on long term evolution (LTE) or LTE-Advanced. To allow client devices to perform synchronization with the network, at least one transmit-receive point (TRP) in each cell of the network transmits periodic synchronization signals. These synchronization signals are detected by the client devices located nearby and used by each client device to identify a proper cell as its serving cell. Hence synchronization allows the client device to acquire a connection to a TRP and track the connection between them for subsequent data communications.
In LTE cellular systems, the synchronization signal comprises a primary synchronization signal (PSS) and a secondary synchronization signal (SSS). PSS and SSS are each transmitted on a unique orthogonal frequency division multiplexing (OFDM) symbol within each periodicity, i.e. within each 5 ms. There are 3 PSSs and 168 SSSs, jointly used to carry 3×168=504 cell identities (IDs). The 168 SSSs are further scrambled by the PSS sequence index, and also scrambled to indicate the first and second half frame timing. Different PSS and SSS sequence pairs carry different cell IDs and are transmitted by TRP(s) in different cells. The client device first acquires coarse time and frequency synchronization as well as an index carried in PSS, NID(2)∈{0, 1, 2}, by detecting PSS in the time domain. The client device then acquires an index carried in SSS, NID(1)∈{0, 1, 2, . . . , 167}, by detecting SSS in the frequency domain. The cell ID is then given by NID=3 NID(1)+NID(2). Specifically, the PSS sequences are constructed based on a length-63 Zadoff Chu (ZC) sequence with three different root indices, and the SSS sequences are constructed by the interleaved concatenation of two length-31 m-sequences with different cyclic shifts, m0 and m1. These two short m-sequences are further scrambled based on NID(2), i.e. there are 168 SSS sequences associated with each PSS sequence, and the second m-sequence is scrambled based on the cyclic shift of the first m-sequence. The cell ID NID is encoded in the SSS sequences via a unique and reversible mapping between the indices NID(1) and NID(2) and the cyclic shifts m0 and m1.
The 3rd generation partnership project (3GPP) is currently working on defining a New Radio (NR) access technology. It has been agreed that synchronization in NR should use 3 NR PSS sequences based on a pure binary phase-shift keying (BPSK) modulated m-sequence with 3 different cyclic shifts. In addition, the number of NR SSSs should be about 1000 after scrambling, i.e. each PSS sequence should correspond to around 333 SSS sequences. Hence, with the 3 NR PSSs around 3×333≈1000 cell IDs can be provided, which is approximately two times the number of cell IDs provided in LTE.
The current LTE SSS design, which concatenates two short m-sequences, suffers from a high risk of cross-correlation as there exist many SSS sequence pairs for which one of the two short m-sequences has the same cyclic shift. This high risk for cross-correlation may cause a high probability of incorrect cell ID detection, especially during hand-over procedure.